A number of medical conditions such as, for example, compression of spinal cord nerve roots, degenerative disc disease, herniated nucleus pulposus, spinal stenosis and spondylolisthesis can cause severe back pain. Intervertebral fusion is one surgical method for alleviating back pain. In a posterior lumbar interbody fusion (“PLIF”) procedure, two adjacent vertebral bodies are fused together by removing the affected intervertebral disc and inserting posteriorly one or more implants on either side of the midline of the spine that would allow for bone to grow between the two adjacent vertebral bodies to bridge the gap left by the removed intervertebral disc.
One variation to the traditional PLIF technique is a transforaminal posterior lumbar interbody fusion (“T-PLIF”) procedure. Pursuant to this procedure, an implant is inserted into the affected disc space via a unilateral (or sometimes bilateral), posterior approach, offset from the midline of the spine, by removing portions of the facet joint of the vertebrae. The T-PLIF procedure avoids damage to nerve structures such as the dura, cauda equina and the nerve root, but the resulting transforaminal window available to remove the affected disc, prepare the vertebral endplates, and insert the implant is limited laterally by soft tissue and medially by the cauda equina.
A number of different implants have been specifically developed for use in connection with the PLIF and T-PLIF procedures with varying success. These include titanium or polymer cages and allograft solid bodies. For example, U.S. Pat. No. 6,719,794 to Gerber et al. discloses, inter alia, an intervertebral implant for use in aT-PLIF procedure made from one or more pieces of allograft bone. The multi-piece implant is joined together by a plurality of interlocking surfaces. The implant may further include one or more pins for securing the implant together. However, because of the interlocking surfaces and pins, the intervertebral implant is formed as a solid body implant (e.g., the implant does not contain any through bore for receiving bone graft material). That is, because of the complexities of forming and machining implants from allograft bone as compared to forming and machining implants from a metal, polymer, etc., manufacturers have been unable to manufacture implants sized and configured for PLIF and T-PLIF procedures that include one or more through bores for receiving bone graft material to facilitate bone fusion.
Moreover, known multi-piece allograft implants are generally initially joined together and then the joined pieces are shaped, this may result in less control and in reduced size potential.
There is a need for an improved intervertebral implant made from multiple pieces of allograft bone, wherein the pieces are joined together to enable the implant to remain assembled in situ, structurally support the required spinal loads and preferably to also contain one or more through-bores for receiving bone graft material to facilitate bone fusion of the adjacent vertebrae bodies.